1. Field of the Invention
This invention relates to a method of treating photoresist materials applied on semiconductor wafer and, more particularly, to a method of treating the photoresist materials employing heating and ultraviolet irradiation.
2. Description of the Prior Art
In the manufacture of semiconductor devices, a process of forming a photoresist pattern is executed in the sequence of the application of a photoresist material, prebaking, exposure, development and postbaking, when roughly divided. Thereafter, ion implantation, or plasma etching of a silicon oxide film, a silicon nitride film, an aluminum film, etc., which are formed beforehand on the surface of a semiconductor wafer prior to the application of the photoresist material, is executed by using the photoresist pattern. On the occasion, it is preferable that the photoresist has a high heat resistance, since the temperature thereof rises in the process of ion implantation, which it is requisite that the photoresist shows a durability not allowing "film erosion" in the process of plasma etching. However, a photoresist material of high resolution, which has been used in recent years as semiconductor devices are highly integrated and made highly fine, is of a positive type, and the photoresist of this type is generally inferior in the heat resistance to the one of a negative type.
With a view to enhancing the heat resistance and plasma resistance of the photoresist, examinations and studies are made on a method in which a photoresist material is heated gradually to an elevated temperature for a sufficient time in postbaking, and a method in which ultraviolet radiation are applied to a photoresist pattern after development thereof. However, the former method has a fault that it fails to ensure satisfactory heat resistance and plasma resistance and requires to expose the photoresist to ultraviolet radiation for a considerably long time. The latter method, on the other hand, has a fault that, although the heat resistance temperature thereof is raised by exposing the photoresist to ultraviolet radiation, ultraviolet rays do not penetrate to the depth of a film of photoresist material when it is thicker, which results in an insufficient improvement in the heat resistance of the entire film photoresist material and reguires a considerably long exposure time.
In view of these faults, a method of combination of "heating" with "ultraviolet irradiation" has been proposed recently, as is disclosed in Japanese Unexamined Patent Publication ("KOKAI KOHO" in Japan)60-45247 (U.S. Application Ser. No. 497,466) titled "Hardening of Photoresist and Apparatus",for instance. This method, however, is unable to meet the demands for improvements in productivity, throughput,etc. since the temperature of the photoresist is raised below the flow temperature of the photoresist at the time the temperature of the photoresist is raised and then the photoresist ie exposed to ultraviolet radiation, and thus the faults in the aforesaid prior-art still appear as problems therein.
As described above, the prior-art methods for particular treatments of the photoresist ,such as hardening the photoresist materisls employing ultraviolet irradiation, have left unsettled the problems that a long exposure time for ultraviolet radiation is required for the treatment and that the improvement in the heat resistance is insufficient in the base portion of the photoresist film when this film is thick, though they have been able to achieve some improvements in the heat resistance and the plasma resistance. In other words, there has been left unsettled a problem that the whole treatment of the photoresist can not be performed in an organic and effective manner.